Lori Dengler | The ‘gateway’ of earthquakes and what it means for earthquake risk – Times-Standard

“Los Angeles on earthquake alert as fault lines reach highest stress levels in history,” read a New York Post headline on July 5. The New York Post certainly tends to exaggerate, but similar stories have appeared in dozens of national news outlets, several science news sites, and on the front page of this newspaper (July 4). What are these stories based on and is there cause for concern?

In early June, a paper was published in the Journal of Geophysical Research (JGR) on stress levels on faults in the southern San Andreas fault system. The study, led by Liliane Burkhard of the University of Bern in Switzerland, confirms that stress has now reached “high levels” and that two major faults are interacting, sometimes limiting ruptures and other times allowing rupture to move from one fault segment to another, producing an earthquake larger than any quake would have occurred alone. JGR is a prestigious journal, and its publication requires extensive peer review. It was a powerful work and provides new analysis of the relationship between two major faults in Southern California.

Fault rupture is something we cannot observe directly. Earthquakes occur at depths ranging from miles to several hundred miles below the surface in areas where there are no direct sensors. Our understanding of earthquake causes and how rupture occurs is based on surface observations, laboratory studies, and modeling. Unlike classroom models or some animations, the bug does not slide all at once, as one side moves instantaneously relative to the other. A rupture always starts at some point and continues to grow in one or two directions, displacing rocks on both sides as they grow.

Faults are weaker areas in the Earth that have produced earthquakes frequently in the past but are by no means uniform. In the case of long fault systems like the San Andreas, they are intersected by many different types of rock of varying strength. Fault lines on a map may appear smooth and straight, but in reality they are complex and contain jagged spots and bends. The rocks on either side of the fault are not only held together by the intense pressure of the ground around them, but over the years since the last rupture, the area may have been partially recrystallized. Changes in temperature, pressure, and fluids in the crack zone affect the strength of different zones.

Stronger stains are called roughness. Thorne Lai and Hiro Kanamori developed the earthquake roughness model in 1981, defining roughness as a strong patch holding a fault together. For a large earthquake to occur, the pressure built up on the fault must exceed the tensile strength. It’s a bit like a zipper. It takes a certain amount of pulling to start moving a zipper, and if you have a sticky zipper, you must gain momentum to move it through difficult spots. Lai and Kanamori initially applied the idea to subduction zones where the largest earthquakes occur, but soon it was applied to all types of faults. In between fluctuations, small earthquakes occur, but to get “big earthquakes” you need to break one or more of these strong spots. Weak points are the starting and stopping points for major earthquakes, and it is also important to consider their role in controlling when an earthquake stops as well as when it starts.

Knowing where variations exist, how strong they are, and how close regional stress is to exceeding their strength is one of the fundamental problems of seismic hazard analysis. There are many tools in a seismologist’s bag to find. The location, size and frequency of past earthquakes is a good starting point. For example, the San Andreas Fault has well-defined segments with distinctly different behavior. There is an area in central California that is constantly creeping and producing only small earthquakes and other parts that are currently stuck but producing large, widely spaced earthquakes.

A recent JGR paper delves into this last category in more detail by looking at past earthquakes and modeling regional stresses for the two most important faults in the southern San Andreas transform fault system: the main trunk of the San Andreas Fault that runs to the north of the Los Angeles Basin, through the San Bernardino, and southeastern Salton Sea and the San Jacinto Fault that parallels it to the west. They are the major players when it comes to the relative movement between the Pacific and North American plates in the southern California region. Both of these faults have produced earthquakes in the mid- to upper-M7 range, but have remained remarkably devoid of major events for more than a century. They are also closely linked, as the San Jacinto Fault branches off from the San Andreas at the Cajon Pass.

The instrumental record of earthquakes in California goes back only 130 years. This record can be traced back a little further by looking at printed newspapers of the time, but the details of older earthquakes require paleoseismological investigations. There have been numerous investigations into trenches along both the southern San Andreas and San Jacinto fault, documenting major faults that spanned nearly 4,000 years. A trench can’t tell you the exact day or magnitude of an earthquake, but it can provide reasonable estimates.

Burkhard’s team took data from trench investigations along several parts of both faults and combined them with other estimates of slip rates and stress parameters to model how the history of earthquakes in the past millennium changed the regional stresses and relative vulnerabilities of the two faults to failure. Their work has identified one point as an important turning point in determining the potential length of a rupture.

The Cajon Pass in San Bernardino County not only separates the San Bernardino Mountains from the San Gabriels Mountains, but also marks the point where the San Jacinto Fault bifurcates from the main San Andreas. Burkhard’s team calls it an “earthquake gateway.” It is a three-way portal, sometimes allowing an earthquake starting in San Andreas to pass to the north or south of the portal, sometimes allowing the San Andreas-San Jacinto fault to rupture, and sometimes stopping rupture from any cold direction. It’s a new term to me – a spot that sometimes stops the tear and other times allows it to spread through it. I think it’s a special kind of sharpness that, depending on the pressure, may prevent rupture or allow it to continue. There is some historical evidence. In 1857, a 7.9 magnitude earthquake tore 225 miles of the San Andreas Fault from Parkfield to Wrightwood, stopping at Cajon Pass. Forty-five years ago, an earthquake of about 7.5 magnitude is believed to have started near Wrightwood and ruptured to the north and on the San Andreas Main but also along much of the San Jacinto Fault to the south. In 1812, the “gate” appears to have been opened, but it was closed in 1857.

The crux of the Cajon Pass Gateway story lies in the three adjacent fault segments, the northern section of the San Joaquin Fault (SJB), and the two sections of the San Andreas above and below that bend (MOS and NSB!). Researchers have calculated the pressure on these parts since 1100 AD, and as earthquakes occur, the pressure changes. There are 12 major earthquakes in this time window, five of which allowed rupture to pass through the gate from the main San Andreas to the north to the south (or vice versa), one that allowed rupture in all three segments, and six where the gate appears to have closed and the rupture stopped at Cajon Pass.

Modeling shows that the current pressure level on MOS, the northern part of San Andreas, has reached an all-time high, surpassing the 1856 value before it exploded. The San Jacinto sector also peaked in this time window. NSB!, the part of San Andreas south of the pass, reached its second highest value. The authors explain that this does not constitute an earthquake prediction or that a large earthquake is likely to occur within days or months, but that “given the time that has passed since these faults ruptured, the probability of an earthquake in the near future is high,” and that high levels in all three segments indicate that the gate is currently open and extension of all three ends of the Cajon Corridor is possible.

The authors acknowledge several uncertainties in their models. While past earthquake dates from trench studies are fairly robust, estimating strength involves a greater degree of error. The study simplifies fault material properties and ignores the potential influence of other faults that shape the larger plate boundary environment, either within the San Andreas system or the eastern California shear zone, which has produced major earthquakes recently. The long time that has passed since the last major earthquake in the southern San Andreas has led most seismologists to consider it more vulnerable than the northern section for many years. The new study identifies the threat and suggests a mechanism for why some ruptures are much longer than others.

For a more detailed explanation of the stress story, see https://temblor.net/earthquake-insights/southern-california-cajon-pass-earthquake-gate-17272/, full publication is at https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2025JB033213

Lori Dengler is professor emeritus of geology at Cal Poly Humboldt and an expert in tsunami and earthquake hazards. Questions or comments about this column, or want a free copy of the “Living on Shaky Ground” preparedness magazine? Leave a message at 707-826-6019 or email [email protected].

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